The present invention relates to a process for the production of porous electrodes, in which a porous metal layer is produced on a framework-forming metallic support having adhesion-promoting surface roughness, is provided with an electrochemically deposited metal in the pores and, if necessary, is activated by treatment with alkali. The present invention further relates to a porous electrode produced by said process.
Active electrodes at which only low overvoltages occur constitute one of the most important preconditions for an economical procedure in electrochemical process engineering. In the case of alkaline electrolyses, such as the electrolysis of an alkali metal chloride or the electrolysis of water, active electrodes based on Raney nickel are usually used. In addition to low overvoltages, such electrodes are also required to have other properties, in particular:
sufficient mechanical strength of the catalyst layer; PA0 economical production even of large units; PA0 applicability with "zero-gap" cell designs (with "zero spacing" between diaphragm and electrode); PA0 homogeneous current density distribution in "zero-gap" cells; and PA0 low-loss transfer of electric charge between the support and the catalyst. PA0 A spreadable paste of a powder mixture of Ni/Al and Ni in 50% of alcohol and 1% of methylcellulose is applied to a sheet metal support and dried. The sheet coated in this manner is then rolled down to about 50% in a cold rolling mill, so that the catalytic powder layer is highly compacted and mechanically bonded to or in the matrix. The powder is subjected to reductive welding by brief annealing at 700.degree. C. in a H.sub.2 atmosphere. This results in a catalyst layer which can be activated and which adheres firmly to the electrically conductive, mechanically stable electrode support.
There are already various known processes for the production of such electrodes. Essentially, an Ni/Al or Ni/Zn alloy, which can be activated, is applied in such processes to an electrically conductive support, from which alloy the soluble component (Al, Zn) is removed by subsequent treatment with an alkali. As a result, a catalytically active Ni structure (Raney nickel) remains. However, the electrodes obtained by the known processes are not completely satisfactory in one respect or another.
For example, according to E. Justi and A. Winsel ("Kalte Verbrennung" [Cold Combustion], Franz Steiner Verlag, 1962, Chapter 4.1), a sintered self-supporting catalyst electrode is produced by a compression or rolling process coupled with a sintering process. However, the said electrode has insufficient mechanical strength at small layer thickness and can be produced only in relatively small dimensions.
Electrodes produced by electrodeposition from suspension (British Pat. No. 2,015,032; U.S. Pat. No. 4,302,322) can be produced only in relatively small units since the electrically conductive suspensions permit regular deposition only at low substrate heights. Moreover, it is impossible to achieve a sufficiently high catalyst concentration with this technique.
Intermetallic diffusion or electrodeposition of an Ni/Zn alloy (U.S. Pat. No. 4,240,895; German Pat. No. 3,330,961) gives electrodes whose structure is not very suitable for low-loss charge transfer.
By plasma spraying ("Hydrogen Energy Progress" V by T. N. Veziroglu and J. B. Taylor (Editors); Pergamon Press, New York, page 933), it is scarcely possible to produce uniform electrodes in the industrially relevant size.
Technically the most mature process is that of reductive powder plating (German Offenlegungsschrift No. 2,829,901; Chem.-Ing.-Technik 5 (1980), 435), which is based on the following principle:
Although electrodes of this type have excellent catalytical activity and mechanical strength, only continuous ("solid") smooth electrodes can be produced, owing to the necessary deformation of the sheet metal support. However, such geometric structures are not very suitable in the "zero-gap" configuration in gas-evolving electrochemical reactions. The geometric form of a perforated metal sheet or expanded metal is known to be necessary for this purpose.
Finally, German Pat. No. 2,914,094 of the Applicant describes a process in which a porous electrode layer is formed on a metal support, such as nickel net or iron net, by sintering an applied suspension of nickel powder, or powder containing a nickel alloy, and pore-forming substances, on which electrode layer a nickel/zinc alloy is deposited electrolytically. Finally, zinc is dissolved away from this electrochemically coated sintered element by immersion in a alkali, which can, if necessary, be carried out in situ when the electrodes are used.
With such electrodes, too, marked overvoltages are still measured.